1. Field of the Invention
The present invention relates generally to an improved thermoelectric module which exhibits superior high temperature performance characteristics and mechanical strength and a method of producing the thermoelectric module.
2. Description of the Related Art
Thermoelectric modules are solid-state devices which are used as either heat pumps or electrical power generators and have found applications in waste heat recovery and temperature control. In an open circuit mode, the thermoelectric module acts like a thermocouple and if a temperature gradient is maintained across the module, an electrical potential results across its terminals which is a function of the temperature difference. On the other hand, if the thermoelectric module is connected to a DC power source, heat will be absorbed at one end of the device, cooling it, while heat is rejected at the other end, where the temperature rises.
Thermoelectric modules are formed from P-type semiconductor elements and N-type semiconductor elements electrically connected in series and the modules are arranged thermally in parallel during use. The semiconductor elements typically are soldered to conductors using different types of solder compositions. Haba, U.S. Pat. No. 3,079,455, discloses a low temperature solder composition formed of tin, antimony and bismuth with the bismuth being provided therein in an amount of from 40 to 50 weight % and the antimony being present in an amount of from 1.5 to 3.5 weight %.
WO 97/13283 discloses a solder composition used in the formation of thermoelectric modules which contains about 50 to 99 weight % bismuth and about 50 to 1 weight % antimony and further discloses that bismuth-tin solder compositions, such as disclosed in U.S. Pat. No. 3,079,455, are not serviceable at temperatures substantially above 80.degree. C. because the tin in the solder tends to diffuse rapidly into the semiconductor elements and into the crystal lattice of the semiconductor elements to act as a dopant or react with the material of the semiconductor elements and that the tin also forms a film over the surface of the material adjacent to the soldered ends where it acts as a resistor across the elements and causes a voltage drop and/or a short. This reference further discloses that Gelb et al, U.S. Pat. No. 4,855,810, sought to overcome the problems of tin diffusion and resistor formation by replacing the tin-based solder with a lead-antimony solder composition but that at elevated temperatures, the lead also diffused and reacted with the thermoelectric semiconductor material to form a region of poor thermoelectric performance.